Radar systems

ABSTRACT

A within-pulse radar having an array of aerial elements each of which feeds signals to a storage filter via a frequency changing stage, in which the outputs from alternate filters are combined in one channel and the outputs from the remaining filters are combined in another channel which includes a switchable phase shifter which introduces a phase shift of 0° or 180° and which is switched during each pulse length, results in improved separation of scanning and grating lobes. The outputs from the two channels are combined for utilization.

This invention relates to radar systems and more specifically to thoseemploying what is now known as "within-pulse" scanning. For brevity ofreference such Radar systems will hereinafter be referred to as"within-pulse radars."

In a within-pulse radar, a pulse of pre-determined length (t) isperiodically transmitted with a pulse repetition period (T) which islong in relation to the pulse length and is preferably and in thesimplest case, a large multiple of the pulse length, e.g., t might be2.5 μ sec. and T might be 2.5 m sec. The transmitted pulse illuminatesthe volume to be surveyed. The receiving equipment operates to quantiseboth target range and target direction information, the small zone ofrange -- i.e., the "range zone" -- in which a reflecting target ispresent being determined by determining the time after the commencementof a pulse period T at which the receiver responds to an echo from thetarget, this time being measured in terms of the number of pulse lengthst equal to the elapsed time between the commencement of a pulse periodand the instant of receiver response. Target direction information (alsoquantized) is given in terms of the time within the pulse length tcorresponding to the target range zone in which maximum receiverresponse occurs. Thus, in the case of a radar designed to survey asector extending from 0° to 90° and made up of 90 distinguishable valueseach of an angle of 1°, a target present in (say) the 40th range zoneand in a direction of 30° would produce a receiver response during the40th pulse length time t from the beginning of a pulse period and onethird of the way along that pulse length time.

The nature of the invention will be more clearly understood by referenceto the accompanying drawings, of which

FIG. 1 shows diagrammatically and sufficiently for the present purposeof explanation, part of the receiving portion of a typical knownwithin-pulse Radar.

FIGS.. 2 and 3 represent schematically the grating lobes resulting fromtwo different orientations of the scanning lobe;

FIGS. 4 and 5 show part of the receiver portion of two embodiments ofthe invention; and

FIG. 6 represents diagrammatically a radar installation incorporating aheight or elevation-finding radar in accordance with the invention.

Referring to FIG. 1 the receiving aerial array consists of a largenumber of aerial elements A₁ to A_(n) which may be of any suitable knowntype, e.g., as shown, radio horns, arranged side by side and each havingan angle of view at least equal to the surveillance angle (e.g., 90°).Each aerial element feeds into one of the similar units F₁ to F_(n) eachof which includes a frequency changing stage and such amplificationmeans as may be required. The local oscillations fed to the differentfrequency changers are, however, not the same but differ from oneanother by the reciprocal 1/t of the pulse length t. The localoscillations are produced by means (not shown) and are applied atterminals LO₁ to LO_(n). Thus, if the local oscillation applied to thefrequency changer at F₁ is f_(o) those applied to the frequency changersat F₂ F₃ . . . F_(n) could be f_(o) - 1/t; f_(o) - 2/t; . . . f_(o) -n/t respectively. Each unit F₁ to F_(n) feeds into a filter S₁ to S_(n)having a bandwidth of 1/t and the outputs of all these filters arecombined and taken off for utilization and information extraction in anyknown desired manner. As the present invention is not concerned withsuch utilization and information extraction, no description thereof isdeemed necessary herein.

With an arrangement as shown in FIG. 1 the array will produce a sharplydirectional scanning lobe which swings over an angle θ, this angle,which is herein termed "the scanning angle," depending on the wavelengthλ and the separation d between the centers of adjacent aerial elementsas given by the equation:

    Sin θ/2 = λ/2d

the bisector of the angle θ being perpendicular to the plane of thearray. However if, as is normally the case in practice, θ is less than180°, the array will produce, in addition to the lobe already mentionedand herein referred to as the scanning lobe, at least one additionallobe which sweeps in the same direction as the scanning lobe. Theseadditional lobes are known as and will herein be termed "grating" lobes.The number of grating lobes depends upon the aerial element spacing,increasing therewith, and because of the desirability of reducing thenumber of aerial elements and signal channels fed individually therebyas much as possible, there are usually, in a practical case, severalgrating lobes. Thus, to take a random practical example, a known radaras illustrated by FIG. 1, wherein the aerial element spacing is 2.2λ andθ is 26°, there will be, when the scanning lobe direction isperpendicular to the plane of the array, four grating lobes, two on oneside of the scanning lobe at angles of (approximately) 27° and 66°thereto and two on the other side of the scanning lobe also at angles of(approximately) 27° and 66° thereto. This is illustrateddiagrammatically in FIG. 2 of the accompanying drawings in which AArepresents the plane of the aerial array, SL represents the scanninglobe and G1 to G4 the grating lobes. When, in scanning, the scanninglobe SL has reached the end of the angle of scan, i.e., is at 13° to theperpendicular to the array, then, as shown in FIG. 3 of the accompanyingdrawings, there will be three grating lobes of which G1 is just enteringthe scan angle θ -- i.e., is at 13° to the perpendicular (P in FIG. 3)to the array -- and the other two, G2 and G3 are on opposite sides ofthe perpendicular, at (approximately) 43° thereto. Thus, in thisparticular example (which is, of course, only one of many) a gratinglobe will begin to scan across the scanning angle θ just when thescanning lobe is leaving it.

As will now be apparent, unwanted scanning lobes are a source ofambiguity and difficulty. Especially is this so if an attempt is made touse a within-pulse radar to scan in the vertical plane for ascertainingthe elevation or height of a target instead of in the horizontal planeto determine its azimuth or bearing. Indeed the difficulties are suchthat, so far as the present applicants are aware, within-pulse radarshave not, in practice, yet been able to be applied satisfactorily fortarget elevation and height determination for the difficulties in suchan application are even greater than they they are in the case ofscanning in the horizontal plane, for the required angle (θ) ofsurveillance is usually small and extends from the horizontal, or nearhorizontal, upwards. Suppose it were required, for target elevationdetermination, to sweep over an angle of 26° extending upwards from thehorizontal. For such a purpose the aerial array would be erected withits plane at 13° to the vertical so that the perpendicular to the arraywould be inclined upwards at 13° to the horizontal. Because of groundreflection and similar effects any grating lobe anywhere near thehorizontal, even though its theoretical direction was inclined downwardsbelow the horizontal to a quite substantial extent, would be liable toresult in reflected interfering and ambiguity causing signals comingback into the receiving array. In addition -- and this difficultyapplies to scanning in the horizontal plane and in the vertical planealike -- it is in practice difficult to the point of impossibility tolimit the transmitter illumination of targets really sharply to adesired angle of surveillance. If the illuminated angle is to cover thewhole desired angle of scan it will inevitably cover, to some extent,more than that angle if only by a few degrees on each side of thelimiting radii. Accordingly a grating lobe directed outside the angle ofscan, but near the limits thereof, will be liable to produce returnedreceived target signals with consequent interference and ambiguity.

The present invention seeks to overcome the foregoing difficulties andto provide improved within-pulse radars, and especially improvedwithin-pulse radars which can be employed satisfactorily for targetheight or elevation determination, which shall be such that undesiredgrating lobes shall be spaced angularly so far from a desired scanninglobe that, when the latter is scanning across a desired angle of scan,there is no grating lobe in that angle or sufficiently near the limitsof that angle to cause ambiguity or interference.

According to this invention a within-pulse radar includes an aerialarray consisting of aerial elements so spaced from one another assubstantially to satisfy the equation Sin θ/2 = λ/2Kd; means forderiving, from the signals received upon the different elements, signalswhich differ from one another in frequency by 1/Kt; K signal channelsfed with the derived signals, each of said channels being fed withsignals derived from 1/K of said elements, the signals derived fromadjacent elements being fed cyclically to different channels; phaseswitching means actuated at least once during each pulse length forsimultaneously switching the phases of the signals in the channels insuch manner that, at each switching operation, the phase in the channelswitched thereby shifts by 2π/K or a multiple thereof, the shiftssimultaneously produced in the different channels being different; andmeans for combining the signals obtained from the channels after phaseswitching to provide signals for utilization and target informationdetermination: where θ is a pre-determined angle of scan, λ is the wavelength, d is the element spacing, t is the pulse length, and K is aninteger.

Preferably and in the simplest case K is 2. It may, however, be a higherinteger, for example 3.

According to a feature of this invention a within-pulse radar comprisesan aerial array consisting of aerial elements spaced so as substantiallyto satisfy the equation Sin θ/2 = λ/4d; as many frequency changers asthere are elements each fed from a different element; means for applyingto the frequency changers fed from adjacent elements local oscillationfrequencies differing by one-half the reciprocal of the pulse length,all the local oscillation frequencies being different; as many filtersas there are frequency changers each fed from a different frequencychanger; two signal channels, one fed from the filters following thefrequency changers fed from alternate elements and the other fed fromthe remaining filters; a phase shifter providing a phase shift of π;means for switching said phase shifter into or out of one of thechannels at least once in each pulse length; and means for combining thesignals from the two channels to provide signals for utilisation andtarget information determination. AS will be seen, in a radar inaccordance with this feature of the invention the above mentionedinteger K is 2.

As already stated the invention is particularly applicable to andintended for providing target elevation or height information and lendsitself admirably to the provision of an elevation or height findingwithin-pulse radar in an otherwise ordinary known radar installation inwhich target azimuth and range determination is obtained in accordancewith ordinary known practice, e.g., in which azimuth scanning iseffected by an aerial system which is rotated in azimuth. In a preferredform of radar installation which includes both a known radar for findingand displaying target range and azimuth, and includes an azimuthrotating aerial system and a height or elevation finding within-pulseradar in accordance with this invention, means, known per se, arepreferably provided for selecting individual displayed targets andgating means actuated by said selecting means are provided in a channelto which the utilisation signals from said within-pulse radar are fed sothat said channel is opened only when signals from a selected target arereceived by the aerial array of the within-pulse radar which is rotatedin azimuth synchronously with the aforesaid azimuth rotating aerialsystem.

The invention is illustrated in FIGS. 4, 5 and 6 of the accompanyingdrawings in which FIG. 4 is a view, similar in nature to that of FIG. 1,showing part of the receiver portion of a preferred embodiment of theinvention; FIG. 5 similarly shows a modification; and FIG. 6 is a muchsimplified diagram of a radar installation which incorporates a heightor elevation finding radar in accordance with the invention.

Referring to FIG. 4 the aerial array comprising the aerial elements A₁to A_(n), the frequency changing units F₁ to F_(n) and the filters S₁ toS_(n) are all as in FIG. 1 except that the element spacing is now chosento satisfy the equation Sin θ/2 = λ/4 d and the frequencies fed to thelocal oscillation terminals LO₁ ; LO₂ ; LO₃ ; LO₄ . . . LO_(n) ₋₁ ;LO_(n) are respectively f_(o) ; f_(o) -1/2 t; f_(o) - 1/t; f_(o) - 3/2;. . . f_(o) - n-1/2t;

and f_(o) - n/2t. The outputs from filters S₁, S₃, S₅ . . . S_(n) ₋₁ arecombined in one channel and those from the remaining alternate filtersare combined in a second channel. In one of the two channels is insertedswitchable phase shifting means capable, in one switching state, ofintroducing a phase shift of 180°. The switchable phase shifting meansare represented diagrammatically in FIG. 4 by a phase shifter PS havingtwo output terminals in the path to one of which there is zero phaseshift and in the path to the other of which there is 180° phase shift, achange over switch SW and a drive member DM for changing over the switchonce during each pulse length t. In practice, of course, the switchmeans would be electronic, e.g., constituted by high speed switchingtransistor circuits. The outputs from the two channels are combined andtaken off for utilisation at U. As in FIG. 1 the bandwidth of each ofthe filters S₁ to S_(n) is 1/t. The phase shifting and switching meansmust be wide band enough to cover the whole band in which the narrowbands from S₁ to S_(n) lie. As will now be seen FIG. 4 shows anembodiment in which the integer K, already mentioned, is 2.

FIG. 5 shows an embodiment in which the integer K is 3. It is thought itwill be found almost self-explanatory from the figure in view of thedescription of FIG. 4 already given. The differences from FIG. 4 are (1)the element spacing is now chosen to satisfy the equation Sin θ/2 =λ/6d; the local oscillation frequencies fed in at LO₁, LO₂, LO₃, LO₄ . .. and so on are now respectively f_(o), f_(o) - 1/3t, f_(o) - 2/3t,f_(o) - 1/t . . . and so on; and there are now three channels, one fedfrom filters S₁, S₄, S₇ . . . and so one, another fed from filters S₂,S₅, S₈ . . . and so on and the third fed from filters S₃, S₆, S₉ . . .and so on. Also there are two switchable phase shifters PS1 and PS2 onein each of two of the channels and each having three output terminalswhich give, in one case phase shifts of 0°, 120° and 240° (counting theterminals from left to right) and in the other case phase shifts of 0°,240° and 120°. There are two ganged switches SW1 and SW2 gang-driven bythe drive means DM. With the switches in the position shown the phaseshifts given by PS1 and PS2 are, respectively, 120° and 240°. Theoutputs from the three channels are combined for utilization. It shouldbe emphasized that although the aerial elements' spacings in FIGS. 1, 4and 5 are shown as alike, this is only to simplify drawing and in factthe spacings are different in the three cases being differently chosento comply with the different equations given for Sin θ/2.

The radar installation represented in FIG. 6 comprises a pulsetransmitter 1 which supplies pulses for transmission to a wave-guide andreflector aerial system 2 or other suitable well known directionalaerial system which is rotated in azimuth and serves both fortransmission and reception. Received echo pulses on this aerial systemare fed to any suitable well known radar receiving equipment 3 anddisplayed on a display unit 4 -- ordinarily a P.P.I. display unit.Associated with the display unit 4 is a marker equipment 5 controlled bya joy-stick control 6 whereby any desired target in the display can, bysuitably positioning the joy-stick control, be marked, e.g., by anelectronically produced ring encircling the selected desired targetrepresentation in the display. As so far described the installation isall well known and may take any suitable form well known per se.

For height finding or elevation determination the receiving part of awithin-pulse radar in accordance with this invention is added. Thiscomprises an aerial array, for example like that representeddiagrammatically in FIG. 4 as comprising elements A₁ to A_(n) spaced asdescribed in connection with FIG. 4. This array is represented as arotatable unit structure to which the reference A is applied in FIG. 6.This array is tilted back at an approximate angle -- for example 13° tothe vertical -- and is rotated about a vertical axis in synchronism withthe rotating aerial system 2. Any known suitable means representedmerely by a synchronizing lead 7 may be used to ensure synchronizedrotation of the two aerial systems 2 and A. Received signals on thearray A are changed in frequency and filtered as already described inconnection with FIG. 4 by apparatus represented by the block FS in FIG.6. The phase shifting and switching apparatus PS and SW of FIG. 4 arerepresented by the block PSW of FIG. 6 and drive circuitry for actuatingthe switch is represented by the block DM of FIG. 4. This circuitry issynchronised with the pulse transmission in any suitable known wayrepresented in FIG. 6 by the lead 8 between the blocks 1 and DM. Thecombined signal output obtained from the output lead U in FIG. 4 is fedto a gating circuit 9 which is controlled by any convenient known gatecircuitry in block 10, this circuitry being in turn controlled by thejoy-stick control 6 in such manner, and also as known per se, that thegate 9 is closed except when signals received from a marked target onthe display unit 4 are being received. The gated output from 9 isintegrated by an integrator 11, and passed to an elevation circuit 12where the height information (if needed) and the elevation informationit contains are extracted by the circuit at 12 and displayed by adisplay unit 13 all as known per se.

The following data is given as a non-limiting practical example of aheight or elevation finding within-pulse radar as illustrated by FIG. 4and which could be included in an installation as represented in FIG. 6:

Scanning angle (θ) 26°

Tilt of aerial array 13° to the vertical

λ = 10 cms (3,000 Mc/s)

t = 5 μ secs

Aerial array: 50 horns spaced at 1.1λ between centers

f_(o) = 2,900 Mc/s

Local oscillator frequency spacing 100 Kc/s

Full band width (to be accommodated by PS and SW) 5 Mc/s

Scanning lobe width 1.3°

I claim:
 1. A within-pulse radar including an aerial array consisting ofaerial elements so spaced from one another as substantially to satisfythe equation Sin θ/2 = λ/2 Kd; means for deriving, from the signalsreceived upon the different elements, signals which differ from oneanother in frequency by 1/Kt; K signal channels fed with the derivedsignals, each of said channels being fed with signals derived from 1/Kof said elements, the signals derived from adjacent elements being fedcyclically to different channels; phase switching means actuated atleast once during each pulse length for simultaneously switching thephases of the signals in the channels in such manner that, at eachswitching operation, the phase in the channel switched thereby shifts by2π/K or a multiple thereof, the shifts simultaneously produced in thedifferent channels being different; and means for combining the signalsobtained from the channels after phase switching to provide signals forutilization and target information determination: where θ is apre-determined angle of scan, λ is the wave length, d is the elementspacing, t is the pulse length, and K is an integer greater than
 1. 2. Awithin-pulse radar including an aerial array consisting of aerialelements spaced so as substantially to satisfy the equation Sin θ/2 =λ/4d; as many frequency changers as there are elements each fed from adifferent element; means for applying to the frequency changers fed fromadjacent elements local oscillation frequencies differing by one halfthe reciprocal of the pulse length, all the local oscillationfrequencies being different; as many storage filters as there arefrequency changers each fed from a different frequency changer; twosignal channels, one fed from the filters following the frequencychangers fed from alternate elements and the other fed from theremaining filters; a phase shifter providing a phase shift of π; meansfor switching said phase shifter into or out of one of the channels atleast once in each pulse length; and means for combining the signalsfrom the two channels to provide signals for utilization and targetinformation determination.
 3. A radar installation comprising a heightor elevation finding within-pulse radar in accordance with claim 2, andfurther including both a known radar for finding and displaying targetrange and azimuth, and an azimuth rotating aerial system and whereinmeans, known per se, are provided for selecting individual displayedtargets and gating means actuated by said selecting means are providedin a channel to which the utilization signals from said within-pulseRadar are fed so that said channel is opened only when signals from aselected target are received by the aerial array of the within-pulseradar which is rotated in azimuth synchronously with the aforesaidazimuth rotating aerial system.
 4. A pulse radar in which an angularsector is scanned electronically over a time period equal to theduration t of an interrogating pulse, comprising a serial array ofspaced aerial elements, means for deriving, from the signals receivedfrom the different elements, signals which differ from one another infrequency, and means for combining the signals obtained from theelements to provide signals for utilization and target informationdetermination, characterized in that the elements are grouped into Kchannels (K being an integer greater than 1), the first channelincluding the first, (1+K)^(th), (1+2K)^(th) . . . aerial elements, thesecond channel including the second, the (2+K)^(th), the (2+2K)^(th)aerial elements and so on, in that the frequencies derived from adjacentaerial elements differ from one another by an amount equal to 1/Kt andin that phase switching means are provided for simultaneously changingthe phases of the signals in the channels during each of said timeperiods of duration t, the phase change in any switched channel being2πn/K, where n is an integer from 1 to K-1, the shift simultaneouslyproduced in the different channels being different.
 5. A pulse radarsystem in which an angular sector is scanned electronically over a timeperiod equal to the duration t of an interrogating pulse, comprising incombination:a serial array of spaced aerial elements adapted to receivetarget reflection signals; means for deriving output signals from thetarget reflection signals received by the different elements, saidoutput signals differing from one another in frequency and being dividedinto K channels, where K is an integer equal to or greater than 2, suchthat the output signals from those elements which are separated by K-1intervening elements of said serial array belong to a common channel;phase shifting means connected to each of said channels other than afirst one thereof for shifting the corresponding output signals by 2π/Kor a multiple thereof at least once during said time period of durationt and such that the phase shift for any two phase-shifted channels isnot the same; and means for combining the outputs of said first channeland of said phase shifting means to provide signals for utilization andtarget information determination.